I tested this before my previous comment and the link setup like that works fine on two of my test installations.
I have created a live test site where you can see this in action, on the homepage I've included a link to Hello World post and you can check it out here https://single.dubajicp.com/

Do you see any console errors on your site when the popup opens?
To open console in Chrome press Ctrl + Shift + J (Windows / Linux) or Cmd + Opt + J (Mac).

Refused to display 'https://www.facebook.com/EXPLORE-Community-1766984080202972?' in a frame because it set 'X-Frame-Options' to 'deny'

And

Mixed Content: The page at 'https://ex.plo.re/new-sir-checkout-page/' was loaded over HTTPS, but requested an insecure resource 'http://rosarealtor.ex.plo.re/?'. This request has been blocked; the content must be served over HTTPS.

I don't see any mixed content errors on that page but I do see Refused error in console.

The thing here is that the response header is set by the domain from which you are requesting the resource and it looks like the source have disallowed loading of the resource in an iframe outside of their domain.

I'm afraid that there's not much you can do here as it depends on the permissions set by the source site :slight_frown:

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